1. Technical Field
The present disclosure relates to multi-color printing systems, and, in particular, to a system and method for determining and correcting color separation registration errors in a multi-color printing system.
2. Description of Related Art
In some multi-color printing systems, such as xerographic multi-color printing systems, multiple color separations are used for marking a substrate, e.g., paper. Usually each separation marks the substrate with only one specific ink (or toner) color; and each separation marks the substrate with a differing color from the other separations, e.g., one separation marks with cyan ink (or toner) while another marks with magenta ink (or toner, hence the adjective “color” in the phrase “color separation”). Also, certain technologies may form a complete multi-color complementary image on an intermediary device before transferring the complementary image to a substrate, e.g., in electrophotographic systems each color separation may cumulatively form a multi-color complementary image on an intermediate substrate before transferring the complementary image to paper.
Although only four different color separations, and hence, four different inks (or toners) are generally used in multicolor printing, a much wider variety of colors are available for perception because of the psychophysical aspects of human vision. For example, small dots of two differing colors located close together may be perceived as an entirely different color when viewed from a sufficient distance because of various aspects of human color perception.
When forming cluster-dot halftone screens, each color separation marks the substrate with discrete shapes, such as dots having a circular or oval shape, or periodic line patterns. This concept is generally known as color half-toning, and involves marking two or more patterned color separations on the substrate. A marked pattern formed by one color separation is generally referred to as a halftone pattern. The selection of the color separation inks (or toners) and the halftone patterns are carefully chosen to achieve a desired visual perception of the desired color.
Many multi-color printing systems utilize cyan, magenta, yellow, and black (also referred to as CMYK) color separations to mark a substrate. The dots may be marked in a dot-on-dot fashion, where each color separation marks dots on top of other dots formed by other color separations; the idea is to achieve a color by superimposing the various separation dots on each other. This creates colors not possible with only one color separation. Dots may also be marked in a dot-off-dot fashion, where the dots of one color separation are placed in the voids of the dots of another color separation. This too can achieve colors not possible by utilizing only one color separation.
However, a subtype category of multi-color printing systems is “highlight color” printing systems, which utilize only two color separations. One of the separations used by highlight color printing systems is usually a black color separation, while the highlight color separation uses a “highlight” color. The highlight color separation usually marks with red ink (or toner), although other colors may be used. Typically, highlight color printing is used to create printed material that is similar in cost to monochrome printers, but has the addition of the highlight color to draw the attention of a reader to a certain item, graphic and/or slogan. For example, many advertisements may have numerous items for sale, but the advertiser may want to highlight a particular item. That item may be printed in the highlight color to grab the attention of the reader. Also, highlight color printing systems are usually capable of a relatively high degree of printing quality at speeds that are comparable to monochrome color printing systems.
However, multi-color printing systems and highlight color printing systems are susceptible to color separation registration errors between color separations due to a variety of mechanical related issues. For example, the color separations may be orientated differently in one direction compared to another direction due to the mechanical tolerances of the color separations; also, vibrations may create localized registration errors by slightly moving a color separation in an undesirable fashion for a short time. For both dot-on-dot and dot-off-dot color-halftone rendering, and highlight color printing, color separation registration errors may cause a significant color shift in the actual printed color that is noticeable by the human eye. Additionally, an unintentional “beating” pattern may appear when viewing a printed image with color separation registration errors. These patterns are called moiré patterns.
Most highlight color printing systems utilize either “image on image” (IOI) highlight color printing or “image-next-to-image” (INI) highlight color printing. Image on image highlight color printing includes marking paper with a black ink (or toner) and a highlight color ink (or toner). The IOI printing system combines the colors of the two color separations to have different shades and/or visual effects on the printed material by combing the two inks or toners visually. For example, it may be easier to create different shades of red by combining black and red cluster-dots. Image-next-to-image highlight color printing usually involves marking separate and distinct regions of a substrate, e.g., paper, with the highlight color and black in the remaining areas. For example, consider a printing system that is tasked to print advertisements about a sale: an advertisement flyer may include several price examples with accompanying graphics all in black, except for the top of the flyer that may have the words “SALE ALL DAY THURSDAY!” printed thereon in the highlight color, e.g., red.
Typically, not all print jobs utilize the highlight color separation of a highlight color printing system. The highlight color separation aspect of the system may be turned off and/or placed in an intermediate standby mode while the “black-only” printing jobs are printed. Additionally or alternatively, the entire printing system may be offline for an extended period, e.g., such as during a weekend and/or vacation period of the operators of the system. Because of these periods of inactivity or quasi-inactivity, the various components and/or elements of a printing system may cool off and misalign resulting in a color separation registration error.
For example, some printing systems utilize a Raster Output Scanner (referred to herein as “ROS”). A ROS may consist of a laser beam source that is sent through various mirrors and lenses and onto a rotating polygon mirror, which is utilized to form an image on a photoreceptor. When “black-only” printing jobs run for an extended period of time, large thermal variation of the black ROS can cause registration errors because of thermal shifts in the mirror or lens mounts. Once the highlight color separation is turned on, the color separation registration error may need correcting. Although utilizing a ROS is an option, a printing system may also utilize a Light Emitting Diode (referred to herein as “LED”) bar to form an image as well. However, both the ROS and LED configurations are susceptible to color separation registration errors.
Consider that on a 600 dpi system a color separation registration error of 8 pixels may be about 338 microns and a color separation registration error of 16 pixels on the same system may be about 677 microns; also consider that on certain highlight color printing systems, the ROS heating and cooling can cause a color separation registration error of greater than 400 microns, while the printing system in aggregate may have a color separation registration errors of greater than 600 microns. The color separations registration errors within a highlight color printing system created by thermal expansion and contraction may be significant enough to increase the need to utilize color separation registration error correcting technologies.
Additionally or alternatively, a photoreceptor belt and/or drum may experience thermal expansion causing additional color separation registration errors to occur. The belt may consist of a coated belt of biaxially-oriented polyethylene terephthalate (boPET) polyester film that is seam welded and is ran in tension on a belt module. The tension roll on the belt module may be between the black and the highlight color separations and may expand or contract as the temperature of the roll and/or belt module varies. However, by utilizing a drive roll that is a Thin Wall Elastomer Drive Roll (TWEDR) some of the color separation registration errors resulting from thermal variation are mitigated. The dynamic nature of the many aspects, components and/or modules of a multi-color printing system has created a need for correcting and/or determining color separation registration errors in a printing system.
A marking technology that mitigates some of these anomalies utilizes rotated cluster dot sets. When using rotated cluster dot sets, the registration error artifacts are more subtle and less detectable by the human eye. However, even in these cases, color separation registration errors may create objectionable images, particularly at the edges of objects that contain more than one color separation. The use of “trapping” areas help to alleviate the effects of registration errors at color boundaries, but the area of the trap is a function of the color separation registration error. Therefore, it is desirable to determine color separation registration errors in order to enhance corrective action to mitigate these and other anomalies.
Various techniques have been used to determine color separation registration errors, such as examining the cluster dots under a microscope. Sometimes, a small patch is printed in the corner of the substrate so that microscope examination may be facilitated. Some of these patches can only be used to measure the color separation registration error in either the fast scan direction (transverse to the longitudinal axis of the photoreceptor belt) or the slow scan direction (parallel to the longitudinal axis of the photoreceptor belt). Multiple patches may also be used to determine the color separation registration error in multiple directions and/or multiple locations.
Control patches are a group of related technologies that are utilized for controlling, adjusting, correcting, and/or determining one or more aspects of a printing system. For example, one kind of patch may facilitate determining color separation registration errors; this kind of patch may be referred to a “color separation registration error” patch or other descriptive name. A “patch image” is the information and/or instructions sent to the color separations. A “patch image as marked on the substrate” is a “patch”. The distinction between a “patch” and a “patch image” is illustrated by noting the difference between what the color separations are instructed to mark (i.e., a “patch image”) and what is actually marked on the substrate (i.e., the “patch”). Thus, a “patch image” as marked on a substrate is a “patch”.
For example, a bitmap file may correspond to a “patch image” and may be configured to determine color separation registration errors. The patch image may be deliberately designed to change in appearance as marked on the substrate as a function of color separation registration error. By examining the patch, the system may be able to determine the color separation registration errors based upon the differences between the bitmap file (the “patch image”, which the separations were instructed to mark) and how the bitmap file was actually marked (i.e., the patch).
With the printer speeds and/or smaller cluster dot sizes now possible there is a need to determine color separation registration errors by utilizing patches and/or patch images to mitigate, correct, or eliminate unwanted artifacts such as moiré patterns, color shifts, and/or other anomalies.